Time division duplex wireless network and associated method using connection modulation groups

ABSTRACT

A wireless network is provided that includes a base station and subscriber stations that communicate with the base station using radio frequency (RF) time division duplex (TDD) signaling. The base station may establish medium access control (MAC) connections with each station. The base station monitors communications with the stations and, in accordance, assigns stations or MAC connections to modulation groups. The base station transmits signals on MAC connections or to stations in a modulation group in adjacent TDD slots within a TDD frame. The base station may receive access requests from the stations, evaluate traffic requirements for the stations, and determine a longest downlink portion for the stations. The base station then allocates downlink and uplink portions of a TDD frame according to the length of the longest downlink portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/088,995, filed Apr. 1, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/667,367, filed on Mar. 24, 2015, which is acontinuation of U.S. patent application Ser. No. 13/437,824, filed onApr. 2, 2012, which is a continuation of U.S. patent application Ser.No. 11/811,945, filed on Jun. 12, 2007, which is a continuation of U.S.patent application Ser. No. 09/948,059, filed on Sep. 5, 2001, issued asU.S. Pat. No. 7,230,931, each of which is hereby incorporated byreference herein in its entirety.

The following applications are hereby incorporated by reference hereinin their entireties: provisional U.S. Patent Application Ser. No.60/262,712 filed on Jan. 19, 2001 and entitled “WIRELESS COMMUNICATIONSYSTEM USING BLOCK FILTERING AND FAST EQUALIZATION DEMODULATION ANDMETHOD OF OPERATION”; provisional U.S. Patent Application Ser. No.60/262,825 filed on Jan. 19, 2001 and entitled “APPARATUS AND ASSOCIATEDMETHOD FOR OPERATING UPON DATA SIGNALS RECEIVED AT A RECEIVING STATIONOF A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM”; provisional U.S.Patent Application Ser. No. 60/262,698 filed on Jan. 19, 2001 andentitled “APPARATUS AND METHOD FOR OPERATING A SUBSCRIBER INTERFACE IN AFIXED WIRELESS SYSTEM”; provisional U.S. Patent Application Ser. No.60/262,827 filed on Jan. 19, 2001 entitled “APPARATUS AND METHOD FORCREATING SIGNAL AND PROFILES AT A RECEIVING STATION”; provisional U.S.Patent Application Ser. No. 60/262,826 filed on Jan. 19, 2001 andentitled “SYSTEM AND METHOD FOR INTERFACE BETWEEN A SUBSCRIBER MODEM ANDSUBSCRIBER PREMISES INTERFACES”; provisional U.S. Patent ApplicationSer. No. 60/262,951 filed on Jan. 19, 2001 entitled “BACKPLANEARCHITECTURE FOR USE IN WIRELESS AND WIRELINE ACCESS SYSTEMS”;provisional U.S. Patent Application Ser. No. 60/262,824 filed on Jan.19, 2001 entitled “SYSTEM AND METHOD FOR ON LINE INSERTION OF LINEREPLACEABLE UNITS IN WIRELESS AND WIRELINE ACCESS SYSTEMS”; provisionalU.S. Patent Application Ser. No. 60/263,101 filed on Jan. 19, 2001entitled “SYSTEM FOR COORDINATION OF TDD TRANSMISSION BURSTS WITHIN ANDBETWEEN CELLS IN A WIRELESS ACCESS SYSTEM AND METHOD OF OPERATION”;provisional U.S. Patent Application Ser. No. 60/263,097 filed on Jan.19, 2001 and entitled “REDUNDANT TELECOMMUNICATION SYSTEM USING MEMORYEQUALIZATION APPARATUS AND METHOD OF OPERATION”; provisional U.S. PatentApplication Ser. No. 60/273,579 filed Mar. 5, 2001 and entitled“WIRELESS ACCESS SYSTEM FOR ALLOCATING AND SYNCHRONIZING UPLINK ANDDOWNLINK OF TDD FRAMES AND METHOD OF OPERATION”; provisional U.S. PatentApplication Ser. No. 60/262,955 filed Jan. 19, 2001 and entitled “TDDFDD AIR INTERFACE”; provisional U.S. Patent Application Ser. No.60/262,708 filed on Jan. 19, 2001 and entitled “APPARATUS, AND ANASSOCIATED METHOD, FOR PROVIDING WLAN SERVICE IN A FIXED WIRELESS ACCESSCOMMUNICATION SYSTEM”; Ser. No. 60/273,689, filed Mar. 5, 2001, entitled“WIRELESS ACCESS SYSTEM USING MULTIPLE MODULATION FORMATS IN TDD FRAMESAND METHOD OF OPERATION”; provisional U.S. Patent Application Ser. No.60/273,757 filed Mar. 5, 2001 and entitled “WIRELESS ACCESS SYSTEM ANDASSOCIATED METHOD USING MULTIPLE MODULATION FORMATS IN TDD FRAMESACCORDING TO SUBSCRIBER SERVICE TYPE”; provisional U.S. PatentApplication Ser. No. 60/270,378 filed Feb. 21, 2001 and entitled“APPARATUS FOR ESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS ACCESSCOMMUNICATION SYSTEM”; provisional U.S. Patent Application Ser. No.60/270,385 filed Feb. 21, 2001 and entitled “APPARATUS FOR REALLOCATINGCOMMUNICATION RESOURCES TO ESTABLISH A PRIORITY CALL IN A FIXED WIRELESSACCESS COMMUNICATION SYSTEM”; and provisional U.S. Patent ApplicationSer. No. 60/270,430 filed Feb. 21, 2001 and entitled “METHOD FORESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS ACCESS COMMUNICATIONSYSTEM.

The present application may share common subject matter and figures withthe following United States Patent Applications, which are incorporatedherein by reference for all purposes as if fully set forth herein:

-   1) Copending Ser. No. 10/042,705, filed on Nov. 15, 2000, entitled    “SUBSCRIBER INTEGRATED ACCESS DEVICE FOR USE IN WIRELESS AND    WIRELINE ACCESS SYSTEMS”;-   2) Ser. No. 09/838,810, filed Apr. 20, 2001, entitled “WIRELESS    COMMUNICATION SYSTEM USING BLOCK FILTERING AND FAST    EQUALIZATION-DEMODULATION AND METHOD OF OPERATION”, now U.S. Pat.    No. 7,075,967;-   3) Ser. No. 09/839,726, filed Apr. 20, 2001, entitled “APPARATUS AND    ASSOCIATED METHOD FOR OPERATING UPON DATA SIGNALS RECEIVED AT A    RECEIVING STATION OF A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM”,    now U.S. Pat. No. 7,099,383;-   4) Copending Ser. No. 09/839,729, filed Apr. 20, 2001, entitled    “APPARATUS AND METHOD FOR OPERATING A SUBSCRIBER INTERFACE IN A    FIXED WIRELESS SYSTEM”;-   5) Ser. No. 09/839,719, filed Apr. 20, 2001, entitled “APPARATUS AND    METHOD FOR CREATING SIGNAL AND PROFILES AT A RECEIVING STATION”, now    U.S. Pat. No. 6,947,477;-   6) Ser. No. 09/838,910, filed Apr. 20, 2001, entitled “SYSTEM AND    METHOD FOR INTERFACE BETWEEN A SUBSCRIBER MODEM AND SUBSCRIBER    PREMISES INTERFACES”, now U.S. Pat. No. 6,564,051;-   7) Copending Ser. No. 09/839,509, filed Apr. 20, 2001, entitled    “BACKPLANE ARCHITECTURE FOR USE IN WIRELESS AND WIRELINE ACCESS    SYSTEMS”;-   8) Ser. No. 09/839,514, filed Apr. 20, 2001, entitled “SYSTEM AND    METHOD FOR ON-LINE INSERTION OF LINE REPLACEABLE UNITS IN WIRELESS    AND WIRELINE ACCESS SYSTEMS”, now U.S. Pat. No. 7,069,047;-   9) Ser. No. 09/839,512, filed Apr. 20, 2001, entitled “SYSTEM FOR    COORDINATION OF TDD TRANSMISSION BURSTS WITHIN AND BETWEEN CELLS IN    A WIRELESS ACCESS SYSTEM AND METHOD OF OPERATION”, now U.S. Pat. No.    6,804,527;-   10) Ser. No. 09/839,259, filed Apr. 20, 2001, entitled “REDUNDANT    TELECOMMUNICATION SYSTEM USING MEMORY EQUALIZATION APPARATUS AND    METHOD OF OPERATION”, now U.S. Pat. No. 7,065,098;-   11) Ser. No. 09/839,457, filed Apr. 20, 2001, entitled “WIRELESS    ACCESS SYSTEM FOR ALLOCATING AND SYNCHRONIZING UPLINK AND DOWNLINK    OF TDD FRAMES AND METHOD OF OPERATION”, now U.S. Pat. No. 7,002,929;-   12) Ser. No. 09/839,075, filed Apr. 20, 2001, entitled “TDD FDD AIR    INTERFACE”, now U.S. Pat. No. 6,859,655;-   13) Copending Ser. No. 09/839,499, filed Apr. 20, 2001, entitled    “APPARATUS, AND AN ASSOCIATED METHOD, FOR PROVIDING WLAN SERVICE IN    A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM”;-   14) Ser. No. 09/839,458, filed Apr. 20, 2001, entitled “WIRELESS    ACCESS SYSTEM USING MULTIPLE MODULATION” (Docket No. WEST14-00026);-   15) Ser. No. 09/839,456, filed Apr. 20, 2001, entitled “WIRELESS    ACCESS SYSTEM AND ASSOCIATED METHOD USING MULTIPLE MODULATION    FORMATS IN TDD FRAMES ACCORDING TO SUBSCRIBER SERVICE TYPE”, now    U.S. Pat. No. 6,891,810;-   16) Copending Ser. No. 09/838,924, filed Apr. 20, 2001, entitled    “APPARATUS FOR ESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS    ACCESS COMMUNICATION SYSTEM”;-   17) Ser. No. 09/839,727 filed Apr. 20, 2001 and entitled “APPARATUS    FOR REALLOCATING COMMUNICATION RESOURCES TO ESTABLISH A PRIORITY    CALL IN A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM”, now U.S. Pat.    No. 7,031,738;-   18) Ser. No. 09/839,734, filed Apr. 20, 2001, entitled “METHOD FOR    ESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS ACCESS    COMMUNICATION SYSTEM”, now U.S. Pat. No. 7,035,241; and-   19) Ser. No. 09/839,513, filed Apr. 20, 2001, entitled “SYSTEM AND    METHOD FOR PROVIDING AN IMPROVED COMMON CONTROL BUS FOR USE IN    ON-LINE INSERTION OF LINE REPLACEABLE UNITS IN WIRELESS AND WIRELINE    ACCESS SYSTEMS”, now U.S. Pat. No. 6,925,516.

The above provisional and non-provisional applications are commonlyassigned to the assignee of the present invention.

TECHNICAL FIELD

The present disclosure is directed, in general, to wireless accesssystems and, more specifically, to a burst packet transmission mediaaccess system for use in a wireless access network.

BACKGROUND

Telecommunications access systems provide for voice, data, andmultimedia transport and control between the central office (CO) of thetelecommunications service provider and the subscriber (customer)premises. Prior to the mid-1970s, the subscriber was provided phonelines (e.g., voice frequency (VF) pairs) directly from the Class 5switching equipment located in the central office of the telephonecompany. In the late 1970s, digital loop carrier (DLC) equipment wasadded to the telecommunications access architecture. The DLC equipmentprovided an analog phone interface, voice CODEC, digital datamultiplexing, transmission interface, and control and alarm remotelyfrom the central office to cabinets located within business andresidential locations for approximately 100 to 2000 phone lineinterfaces. This distributed access architecture greatly reduced linelengths to the subscriber and resulted in significant savings in bothwire installation and maintenance. The reduced line lengths alsoimproved communication performance on the line provided to thesubscriber.

By the late 1980s, the limitations of data modem connections over voicefrequency (VF) pairs were becoming obvious to both subscribers andtelecommunications service providers. ISDN (Integrated Services DigitalNetwork) was introduced to provide universal 128 kbps service in theaccess network. The subscriber interface is based on 64 kbpsdigitization of the VF pair for digital multiplexing into high speeddigital transmission streams (e.g., T1/T3 lines in North America, E1/E3lines in Europe). ISDN was a logical extension of the digital networkthat had evolved throughout the 1980s. The rollout of ISDN in Europe washighly successful. However, the rollout in the United States was notsuccessful, due in part to artificially high tariff costs which greatlyinhibited the acceptance of ISDN.

More recently, the explosion of the Internet and deregulation of thetelecommunications industry have brought about a broadband revolutioncharacterized by greatly increased demands for both voice and dataservices and greatly reduced costs due to technological innovation andintense competition in the telecommunications marketplace. To meet thesedemands, high speed DSL (digital subscriber line) modems and cablemodems have been developed and introduced. The DLC architecture wasextended to provide remote distributed deployment at the neighborhoodcabinet level using DSL access multiplexer (DSLAM) equipment. Theincreased data rates provided to the subscriber resulted in upgradeDLC/DSLAM transmission interfaces from T1/E1 interfaces (1.5/2.0 Mbps)to high speed DS3 and OC3 interfaces. In a similar fashion, the entiretelecommunications network backbone has undergone and is undergoingcontinuous upgrade to wideband optical transmission and switchingequipment.

Similarly, wireless access systems have been developed and deployed toprovide broadband access to both commercial and residential subscriberpremises. Initially, the market for wireless access systems was drivenby rural radiotelephony deployed solely to meet the universal servicerequirements imposed by government (i.e., the local telephone company isrequired to serve all subscribers regardless of the cost to installservice). The cost of providing a wired connection to a small percentageof rural subscribers was high enough to justify the development andexpense of small-capacity wireless local loop (WLL) systems.

Deregulation of the local telephone market in the United States (e.g.,Telecommunications Act of 1996) and in other countries shifted the focusof fixed wireless access (FWA) systems deployment from rural access tocompetitive local access in more urbanized areas. In addition, the ageand inaccessibility of much of the older wired telephone infrastructuremakes FWA systems a cost-effective alternative to installing new, wiredinfrastructure. Also, it is more economically feasible to install FWAsystems in developing countries where the market penetration is limited(i.e., the number and density of users who can afford to pay forservices is limited to small percent of the population) and the rolloutof wired infrastructure cannot be performed profitably. In either case,broad acceptance of FWA systems requires that the voice and data qualityof FWA systems must meet or exceed the performance of wiredinfrastructure.

Wireless access systems must address a number of unique operational andtechnical issues including:

-   -   1) Relatively high bit error rates (BER) compared to wire line        or optical systems; and    -   2) Transparent operation with network protocols and protocol        time constraints for the following protocols:        -   a) ATM;        -   b) Class 5 switch interfaces (domestic GR-303 and            international V5.2);        -   c) TCP/IP with quality-of-service QoS for voice over IP            (VoIP) (i.e., RTP) and other H.323 media services;        -   d) Distribution of synchronization of network time out to            the subscribers;    -   3) Increased use of voice, video and/or media compression and        concentration of active traffic over the air interface to        conserve bandwidth;    -   4) Switching and routing within the access system to distribute        signals from the central office to multiple remote cell sites        containing multiple cell sectors and one or more frequencies of        operation per sector; and    -   5) Remote support and debugging of the subscriber equipment,        including remote software upgrade and provisioning.

Unlike physical optical or wire systems that operate at bit error rates(BER) of 10⁻¹¹, wireless access systems have time varying channels thattypically provide bit error rates of 10⁻³ to 10⁻⁶. The wireless physical(PHY) layer interface and the media access control (MAC) layer interfacemust provide modulation, error correction and ARQ (automatic request forretransmission) protocol that can detect and, where required, correct orretransmit corrupted data so that the interfaces at the network and atthe subscriber site operate at wire line bit error rates.

Wireless access systems, as well as other systems which employ a sharedcommunications media, must also provide a mechanism for allocatingavailable communications bandwidth among multiple transmitting andreceiving groups. Many wireless systems employ either a time divisionduplex (TDD) time division multiple access (TDMA) or a frequencydiversity duplex (FDD) frequency division multiple access (FDMA)allocation scheme illustrated by the timing diagram of FIGS. 10A and10B. TDD 1000 shares a single radio frequency (RF) channel F1 betweenthe base and subscriber, allocating time slices between the downlink1001 (transmission from the base to the subscriber) and the uplink 1002(transmission from the subscriber to the base). FDD 1010 employs twofrequencies F1 and F2, each dedicated to either the downlink 1011 or theuplink 1012 and separated by a duplex spacing 1013.

For wireless access systems which provide Internet access in addition toor in lieu of voice communications, data and other Web basedapplications dominate the traffic load and connections within thesystem. Data access is inherently asymmetric, exhibiting typicaldownlink-to-uplink ratios of between 4:1 and 14:1.

TDD systems, in which the guard point (the time at which changeover fromthe downlink 1001 to the uplink 1002 occurs) within a frame may beshifted to alter the bandwidth allocation between the downlink 1001 andthe uplink 1002, have inherent advantages for data asymmetry andefficient use of spectrum in providing broadband wireless access. TDDsystems exhibit 40% to 90% greater spectral efficiency for asymmetricdata communications than FDD systems, and also support shifting of powerand modulation complexity from the subscriber unit to the base to lowersubscriber equipment costs.

Within the spectrum allocated to multichannel multipoint distributionsystems (MMDS), however, some spectrum is regulated for only FDDoperation. Since the total spectrum allocated to MMDS is relativelysmall (2.5-2.7 GHz, or about 30 6 MHz channels), some service providersmay desire to utilize the FDD-only spectrum, preferably utilizing theTDD-based equipment employed in other portions of the MMDS spectrum.

The wide range of equipment and technology capable of providing eitherwireline (i.e., cable, DSL, optical) broadband access or wirelessbroadband access has allowed service providers to match the needs of asubscriber with a suitable broadband access solution. However, in manyareas, the cost of cable modem or DSL service is high. Additionally,data rates may be slow or coverage incomplete due to line lengths. Inthese areas and in areas where the high cost of replacing old telephoneequipment or the low density of subscribers makes it economicallyunfeasible to introduce either DSL or cable modem broadband access,fixed wireless broadband systems offer a viable alternative. Fixedwireless broadband systems use a group of transceiver base stations tocover a region in the same manner as the base stations of a cellularphone system. The base stations of a fixed wireless broadband systemtransmit forward channel (i.e., downstream) signals in directed beams tofixed location antennas attached to the residences or offices ofsubscribers. The base stations also receive reverse channel (i.e.,upstream) signals transmitted by the broadband access equipment of thesubscriber.

Media access control (MAC) protocols refer to techniques that increaseutilization of two-way communication channel resources by subscribersthat use the channel resources. The MAC layer may use a number ofpossible configurations to allow multiple access. These configurationsinclude:

-   -   1. FDMA—frequency division multiple access. In a FDMA system,        subscribers use separate frequency channels on a permanent or        demand access basis.    -   2. TDMA—time division multiple access. In a TDMA system,        subscribers share a frequency channel but allocate spans of time        to different users.    -   3. CDMA—code division multiple access. In a CDMA system,        subscribers share a frequency but use a set of orthogonal codes        to allow multiple access.    -   4. SDMA—space division multiple access—In a SDMA system,        subscribers share a frequency but one or more physical channels        are formed using antenna beam forming techniques.    -   5. PDMA—polarization division multiple access—In a PDMA system,        subscribers share a frequency but change polarization of the        antenna.

Each of these MAC techniques makes use of a fundamental degree offreedom (physical property) of a communications channel. In practice,combinations of these degrees of freedom are often used. As an example,cellular systems use a combination of FDMA and either TDMA or CDMA tosupport a number of users in a cell.

To provide a subscriber with bi-directional (two-way) communication in ashared media, such as a coaxial cable, a multi-mode fiber (optical), oran RF radio channel, some type of duplexing technique must beimplemented. Duplexing techniques include frequency division duplexing(FDD) and time division duplexing (TDD). In FDD, a first channel(frequency) is used for transmission and a second channel (frequency) isused for reception. To avoid physical interference between the transmitand receive channels, the frequencies must have a separation know as theduplex spacing. In TDD, a single channel is used for transmission andreception and specific periods of time (i.e., slots) are allocated fortransmission and other specific periods of time are allocated forreception.

Finally, a method of coordinating the use of bandwidth must beestablished. There are two fundamental methods: distributed control andcentralized control. In distributed control, subscribers have a sharedcapability with or without a method to establish priority. An example ofthis is CSMA (carrier sense multiple access) used in IEEE802.3 Ethernetand IEEE 802.11 Wireless LAN. In centralized control, subscribers areallowed access under the control of a master controller. Cellularsystems, such as IS-95, IS-136, and GSM, are typical examples. Access isgranted using forms of polling and reservation (based on polled ordemand access contention).

A number of references and overviews of demand access are availableincluding the following:

-   1. Sklar, Bernard. “Digital Communications Fundamentals and    Applications,” Prentice Hall, Englewood Cliffs, N.J., 1988. Chapter    9.-   2. Rappaport, Theodore. “Wireless Communications, Principles and    Practice,” Prentice Hall, Upper Saddle River, N.J., 1996. Chapter 8.-   3. TR101-173V1.1. “Broadband Radio Access Networks, Inventory of    Broadband Radio Technologies and Techniques,” ETSI, 1998. Chapter 7.    The foregoing references are hereby incorporated by reference into    the present disclosure as if fully set forth herein.

In 1971, the University of Hawaii began operation of a random accessshared channel ALOHA TDD system. The lack of channel coordinationresulted in poor utilization of the channel. This lead to theintroduction of time slots (slotted Aloha) that set a level ofcoordination between the subscribers that doubled the channelthroughput. Finally, the researchers introduced the concept of a centralcontroller and the use of reservations (reservation Aloha). Reservationtechniques made it possible to make trade-offs between throughput andlatency.

This work was fundamental to the development of media access control(MAC) techniques for dynamic random access and the use of ARQ (automaticrequest for retransmission) to retransmit erroneous packets. While thework at the University of Hawaii explored the fundamentals of bursttransmission and random access, the work did not introduce the conceptof a frame and/or super-frame structure to the TDD/TDMA accesstechniques. One of the more sophisticated systems developed in the 1970sand in current use is Joint Tactical Information Distribution System(JTIDS). This system was based on the joint use of TDMA and timeduplexing over frequency-hopping spread-spectrum channels. This was theculmination of research to allow flexible allocation of bandwidth to alarge group of users. The key aspect of the JTIDS system was theintroduction of dynamic allocation of bandwidth resources and explicitvariable symmetry (downlink vs. uplink bandwidth) in the link.

IEEE 802.11 Wireless LAN equipment provides for a centrally coordinatedTDD system that does not have a specific frame or slotting structure.IEEE 802.11 did introduce the concept of variable modulation andspreading inherent in the structure of the transmission bursts. Asignificant improvement was incorporated in U.S. Pat. No. 6,052,408,entitled “Cellular Communications System with Dynamically Modified DataTransmission Parameters.” This patent introduced specific burst packettransmission formats that provide for adaptive modulation, transmitpower, and antenna beam forming and an associated method of determiningthe highest data rate for a defined error rate floor for the linkbetween the base station and a plurality of subscribers assigned to thatbase station. With the exception of variable spreading military systemsand NASA space communication systems, this was one of the firstcommercial patents that address variable transmission parameters toincrease system throughput.

Another example of TDD systems is digital cordless phones, also referredto as low-tier PCS systems. The Personal Access Communications (PAC)system and Digital European Cordless Telephone (DECT, as specified byETSI document EN 300-175-3) are two examples of these systems. Digitalcordless phones met with limited success for their intended use aspico-cellular fixed access products. The systems were subsequentlymodified and repackaged for wireless local loop (WLL) applications withextended range using increased transmission (TX) power and greaterantenna gain.

These TDD/TDMA systems use fixed symmetry and bandwidth between theuplink and the downlink. The TDD frame consists of a fixed set of timeslots for the uplink and the downlink. The modulation index (or type)and the forward error correction (FEC) format for all data transmissionsare fixed in these systems. These systems did not include methods forcoordinating TDD bursts between systems. This resulted in inefficientuse of spectrum in the frequency planning of cells.

While DECT and PAC systems based on fixed frames with fixed andsymmetric allocation of time slots (or bandwidth) provides excellentlatency and low jitter, and can support time bounded services, such asvoice and Nx64 Kbps video, these systems do not provide efficient use ofthe spectrum when asymmetric data services are used. This has lead toresearch and development of packet based TDD systems based on Internetprotocol (IP) or asynchronous transfer mode (ATM), with dynamicallocation of TDD time slots and the uplink-downlink bandwidth, combinedwith efficient algorithms to address both best efforts and real-timelow-latency service for converged media access (data and multi-media).

One example of a TDD system with dynamic slot and bandwidth assignmentis the ETSI HYPERLAN II specification based on the Dynamic SlotAssignment algorithm described in “Wireless ATM: Performance Evaluationof a DSA++ MAC Protocol with Fast Collision resolution by ProbingAlgorithm,” D. Petras and A. Kramling, International Journal of WirelessInformation Networks, Vol. 4, No. 4, 1997. This system allows bothcontention-based and contention-free access to the physical TDD channelslots. This system also introduced the broadcast of resource allocationat the start of every frame by the base station controller. Otherwireless standards, including IEEE 802.16 wireless metropolitan networkstandards, use this combination of an allocation MAP of the uplink anddownlink at the start of the dynamic TDD frame to set resource use forthe next TDD frame.

A further improvement to this TDD system was described in “MultipleAccess Control Protocols for Wireless ATM: Problem Definition and DesignObjectives,” O. Kubbar and H. Mouftah, IEEE Communications, November1997, pp. 93-99. This system expanded on the packet reservation multipleaccess (PRMA) method developed in 1989 at Rutgers University WINLAB forATM and IP based transport [see “Packet Reservation Multiple Access forLocal Wireless Communications,” Goodman et al., IEEE Transaction onCommunications, Vol. 37, No. 8, pp. 885-890]. Like PRMA, this systemlogically arranged all the downlink transmissions in the start of afixed duration TDD frame and all uplink transmissions at the end of theTDD frame. This eliminated the inefficiencies in the DCA++ Hyperlan IIprotocol. Adaptive allocation of uplink and downlink bandwidth issupported. The system provided for fixed, random, and demand assignmentmechanisms. Priority is given to quality of service (QoS) applicationswith resources being removed from best efforts demand access users asrequired.

The above-described prior art concern the allocation of services in anindividual sector of a cell. A cell may consist of M sectors, whereineach sector generally covers a 360/M degree arc around the cell site.Each sector serves N_(m) subscribers, where m=1 to M. These referencesdid not expressly provide protocol mechanisms or rules for the operationof a given system.

U.S. Pat. No. 6,016,311 expressly addresses one possible implementationto the TDD bandwidth allocation problem. The system describedcontinuously measures and adapts the bandwidth requirements based on theevaluation of the average bandwidth required by all the subscribers in acell and the number of times bandwidth is denied to the subscribers.Changes to the bandwidth allocation are applied based on a set of rulesdescribed in U.S. Pat. No. 6,016,311. While measurements of multiplesectors are performed and recorded at a central base station controller,no global coordination of bandwidth allocation of multiple sectors in acell or across multiple cells is provided.

Thus, the prior art does not address two very important factors inallocation of bandwidth. First, bandwidth allocation must contemplatestringent bandwidth availability requirements for specific groups ofservices based on planning of the network. For example, considerlife-line toll quality voice service. Toll quality voice requires that asystem guarantee a specific maximum blocking probability for all voiceusers based on peak busy hour call usage. A description of voice trafficplanning is provided in “Digital Telephony—2_(nd) Edition,” by J.Bellamy, John Wiley and Sons, New York, N.Y., 1990. If a TDD system isdesigned to meet life-line voice requirements, the allocation protocolmust be able to rapidly (i.e., less than 100 msec) reallocate bandwidthresources up to the capacity necessary to meet the call blockingrequirements. Another service group example is a guaranteed servicelevel agreements (SLA). Again, bandwidth must be rapidly restored tomeet the SLA conditions. More generally, one may consider G possibleservice groups having a set of weighted priority level and associatedminimum and maximum levels. The weighted priority levels and minimum andmaximum levels may be used to bound the bandwidth dynamics of the TDDbandwidth allocation. Minimum levels set a floor for bandwidthallocation and maximum levels set a ceiling. Then averaging can beapplied.

Second, the TDD bandwidth allocation must consider adjacent andco-channel interference from both modems and sectors within a cell andbetween cells. Cell planning tools can be used to establish therelationships for interference. For systems that operate below 10 GHz,antennas and antenna placement at a cell site will not provide adequatesignal isolation. These co-channel interference issues are welldocumented in “Frequency Reuse and System Deployment in Local MultipointDistribution Service,” by V. Roman, IEEE Personal Communications,December 1999, pp. 20 to 27.

Therefore, there is a need in the art for a fixed wireless accessnetwork that maximizes spectral efficiency between the base stations ofthe fixed wireless access network and the subscriber access deviceslocated at the subscriber premises. In particular, there is a need for afixed wireless access network that implements an air interface thatminimizes uplink and downlink interference between different sectorswithin the same base station cell site. There also is a need for a fixedwireless access network that implements an air interface that minimizesuplink and downlink interference between different cell sites within thefixed wireless access network. More particularly, there is a need in theart for a fixed wireless that efficiently allocates bandwidth toindividual subscribers according to dynamically changing applicationsused by the individual subscribers.

SUMMARY

Aspects of the disclosure may be found in a wireless network thatincludes a base station in wireless communication with subscriberstations using radio frequency (RF) time division duplex (TDD)signaling. For each of the subscriber stations, the base stationestablishes a plurality of medium access control (MAC) connections on aRF link between the base station and the station. The base stationmonitors communication traffic on the MAC connections. Based on themonitored traffic, the base station sets for each MAC connection acontrollable characteristic of the RF communication with the associatedsubscriber station and assigns the MAC connection to a modulation groupbased on the controllable characteristic. The base station furthertransmits signals on MAC connections in a modulation group in adjacentTDD slots within a TDD frame.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure so that those skilled in the artmay better understand the detailed description of the disclosure thatfollows. Additional features and advantages of the disclosure will bedescribed hereinafter that form the subject of the claims of theinvention. Those skilled in the art should appreciate that they mayreadily use the conception and the specific embodiment disclosed as abasis for modifying or designing other structures for carrying out thesame purposes of the present disclosure. Those skilled in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the disclosure in its broadest form.

Before undertaking the DETAILED DESCRIPTION below, it may beadvantageous to set forth definitions of certain words and phrases usedthroughout this patent document: the terms “include” and “comprise,” aswell as derivatives thereof, mean inclusion without limitation; the term“or,” is inclusive, meaning and/or; the phrases “associated with” and“associated therewith,” as well as derivatives thereof, may mean toinclude, be included within, interconnect with, contain, be containedwithin, connect to or with, couple to or with, be communicable with,cooperate with, interleave, juxtapose, be proximate to, be bound to orwith, have, have a property of, or the like; and the term “controller”means any device, system or part thereof that controls at least oneoperation, such a device may be implemented in hardware, firmware orsoftware, or some combination of at least two of the same. It should benoted that the functionality associated with any particular controllermay be centralized or distributed, whether locally or remotely.Definitions for certain words and phrases are provided throughout thispatent document, those of ordinary skill in the art should understandthat in many, if not most instances, such definitions apply to prior, aswell as future uses of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, wherein likenumbers designate like objects, and in which:

FIG. 1 illustrates an exemplary fixed wireless access network accordingto one embodiment of the present disclosure;

FIG. 2 illustrates in greater detail an alternate view of selectedportions of the exemplary fixed wireless access network according to oneembodiment of the present disclosure;

FIG. 3 illustrates an exemplary time division duplex (TDD) time divisionmultiple access (TDMA) frame according to one embodiment of the presentdisclosure;

FIG. 4 illustrates the timing recovery and distribution circuitry in anexemplary RF modem shelf according to one embodiment of the presentdisclosure;

FIG. 5 illustrates an interface tray in an exemplary RF modem shelfaccording to one embodiment of the present disclosure;

FIG. 5A illustrates an exemplary time division duplex (TDD) framesaccording to one embodiment of the present disclosure;

FIG. 5B illustrates an exemplary transmission burst containing a singleFEC block according to one embodiment of the present disclosure;

FIG. 5C illustrates an exemplary transmission burst containing multipleFEC blocks according to one embodiment of the present disclosure;

FIG. 6 is a flow diagram illustrating the adaptive modification of theuplink and downlink bandwidth in the air interface in wireless accessnetwork according to one embodiment of the present disclosure;

FIG. 7 is a flow diagram illustrating the adaptive assignment ofselected link parameters, such as modulation format, forward errorcorrection (FEC) codes, and antenna beam forming, to the uplink anddownlink channels used by each subscriber in the exemplary wirelessaccess network according to one embodiment of the present disclosure;

FIG. 8 is a flow diagram illustrating the adaptive assignment ofselected link parameters to the different service connections used byeach subscriber in the wireless access network according to oneembodiment of the present disclosure;

FIGS. 9A and 9B depict cell and sector layouts for a wireless accesscoverage area according to various embodiments of the presentdisclosure;

FIGS. 10A through 10E are comparative high level timing diagramsillustrating the bandwidth allocation among sectors and cells accordingto the prior art and according to one embodiment of the presentdisclosure;

FIG. 11 depicts in greater detail a frame structure employed within theexemplary bandwidth allocation scheme according to one embodiment of thepresent disclosure;

FIG. 12 is functional diagram of filtering employed for wirelesscommunication within each cell and sector in accordance with oneembodiment of the present disclosure;

FIG. 13 illustrates a spectral response for filtering employed forwireless communication within each cell and sector in accordance withone embodiment of the present disclosure; and

FIG. 14 is functional diagram of filtering employed for wirelesscommunication within each cell and sector in accordance with anotherembodiment of the present disclosure.

DETAILED DESCRIPTION

FIGS. 1 through 14, discussed below, and the various embodiments used todescribe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any suitably arranged wireless access system.

FIG. 1 illustrates exemplary fixed wireless access network 100 accordingto one embodiment of the present disclosure. Fixed wireless network 100comprises a plurality of transceiver base stations, including exemplarytransceiver base station 110, that transmit forward channel (i.e.,downlink or downstream) broadband signals to a plurality of subscriberpremises, including exemplary subscriber premises 121, 122 and 123, andreceive reverse channel (i.e., uplink or upstream) broadband signalsfrom the plurality of subscriber premises. Subscriber premises 121-123transmit and receive via fixed, externally-mounted antennas 131-133,respectively. Subscriber premises 121-123 may comprise many differenttypes of residential and commercial buildings, including single familyhomes, multi-tenant offices, small business enterprises (SBE), mediumbusiness enterprises (MBE), and so-called “SOHO” (small office/homeoffice) premises.

The transceiver base stations, including transceiver base station 110,receive the forward channel (i.e., downlink) signals from externalnetwork 150 and transmit the reverse channel (i.e., uplink) signals toexternal network 150. External network 150 may be, for example, thepublic switched telephone network (PSTN) or one or more data networks,including the Internet or proprietary Internet protocol (IP) wide areanetworks (WANs) and local area networks (LANs). Exemplary transceiverbase station 110 is coupled to RF modem shelf 140, which, among otherthings, up-converts baseband data traffic received from external network150 to RF signals transmitted in the forward channel to subscriberpremises 121-123. RF modem shelf 140 also down-converts RF signalsreceived in the reverse channel from subscriber premises 121-123 tobaseband data traffic that is transmitted to external network 150. In anexemplary embodiment of the present disclosure in which external network150 is the public switched telephone network (PSTN), RF modem 140transmits baseband data traffic to, and receives baseband data trafficfrom, access processor 165, which is disposed in central office facility160 of the PSTN.

It should be noted that network 100 was chosen as a fixed wirelessnetwork only for the purposes of simplicity and clarity in explaining asubscriber integrated access device according to the principles of thepresent disclosure. The choice of a fixed wireless network should not beconstrued in any manner that limits the scope of the present disclosurein any way. As will be explained below in greater detail, in alternateembodiments of the present disclosure, a subscriber integrated accessdevice according to the principles of the present disclosure may beimplemented in other types of broadband access systems. In oneembodiment of the present disclosure, such access systems may includewireline systems (i.e., digital subscriber line (DSL), cable modem,fiber optic, and the like) in which a wireline connected to thesubscriber integrated access device carries forward and reverse channelsignals.

RF modem shelf 140 comprises a plurality of RF modems capable ofmodulating (including up-converting) the baseband data traffic anddemodulating (including down-converting) the reverse channel RF signals.In an exemplary embodiment of the present disclosure, each of thetransceiver base stations covers a cell site area that is divided into aplurality of sectors. In an advantageous embodiment of the presentdisclosure, each of the RF modems in RF modem shelf 140 may be assignedto modulate and demodulate signals in a particular sector of each cellsite. By way of example, the cell site associated with transceiver basestation 110 may be partitioned into six sectors and RF modem shelf 140may comprise six primary RF modems (and, optionally, a seventh spare RFmodem), each of which is assigned to one of the six sectors in the cellsite of transceiver base station 110. In another advantageous embodimentof the present disclosure, each RF modem in RF modem shelf 140 comprisestwo or more RF modem transceivers which may be assigned to at least oneof the sectors in the cell site. For example, the cell site associatedwith transceiver base station 110 may be partitioned into six sectorsand RF modem shelf 140 may comprise twelve RF transceivers that areassigned in pairs to each one of the six sectors. The RF modems in eachRF modem pair may alternate modulating and demodulating the downlink anduplink signals in each sector.

RF modem shelf 140 is located proximate transceiver base station 110 inorder to minimize RF losses in communication line 169. RF modem shelf140 may receive the baseband data traffic from external network 150 andtransmit the baseband data traffic to external network 150 via a numberof different paths. In one embodiment of the present disclosure, RFmodem shelf 140 may transmit baseband data traffic to, and receivebaseband data traffic from, external network 150 through central officefacility 160 via communication lines 166 and 167. In such an embodiment,communication line 167 may be a link in a publicly owned or privatelyowned backhaul network. In another embodiment of the present disclosure,RF modem shelf 140 may transmit baseband data traffic to, and receivebaseband data traffic from, external network 150 directly viacommunication line 168 thereby bypassing central office facility 160.

Central office facility 160 comprises access processor shelf 165. Accessprocessor shelf 165 provides a termination of data traffic for one ormore RF modem shelves, such as RF modem shelf 140. Access processorshelf 165 also provides termination to the network switched circuitinterfaces and/or data packet interfaces of external network 150. One ofthe principal functions of access processor shelf 165 is to concentratedata traffic as the data traffic is received from external network 150and is transferred to RF modem shelf 140. Access processor shelf 165provides data and traffic processing of the physical layer interfaces,protocol conversion, protocol management, and programmable voice anddata compression.

FIG. 2 illustrates in greater detail an alternate view of selectedportions of exemplary fixed wireless access network 100 according to oneembodiment of the present disclosure. FIG. 2 depicts additionaltransceiver base stations, including exemplary transceiver base stations110A through 110F, central office facilities 160A and 160B, and remoteRF modem shelves 140A through 140D. Central office facilities 160A and160B comprise internal RF modems similar to RF modem shelves 140Athrough 140D. Transceiver base stations 110A, 110B, and 110C aredisposed in cells sites 201, 202, and 203, respectively. In theexemplary embodiment, cell sites 201-203 (shown in dotted lines) arepartitioned into four sectors each. In alternate embodiments, sites 201,202, and 203 may be partitioned into a different number of sectors, suchas six sectors, for example.

As in FIG. 1, RF modem shelves 140A-140D and the internal RF modems ofcentral office facilities 160A and 160B transmit baseband data trafficto, and receive baseband data traffic from, access processors in centraloffice facilities 160A and 160B of the PSTN. RF modem shelves 140A-140Dand the internal RF modems of central office facilities 160A and 160Balso up-convert incoming baseband data traffic to RF signals transmittedin the forward (downlink) channel to the subscriber premises anddown-convert incoming RF signals received in the reverse (uplink)channel to baseband data traffic that is transmitted via a backhaulnetwork to external network 150.

Baseband data traffic may be transmitted from remote RF modem shelves140A-140D to central office facilities 160A and 160B by a wirelessbackhaul network or by a wireline backhaul network, or both. As shown inFIG. 2, baseband data traffic is carried between central office facility160A and remote RF modem 140A by a wireline backhaul network, namelywireline 161, which may be, for example, a DS3 line or one to N T1lines. A local multipoint distribution service (LMDS) wireless backhaulnetwork carries baseband data traffic between central office facilities160A and 160B and remote RF modem shelves 140B, 140C, and 140D. In aLMDS wireless backhaul network, baseband data traffic being sent toremote RF modem shelves 1408, 140C, and 140D is transmitted by microwavefrom microwave antennas mounted on transceiver base stations 110A, 110C,and 110? to microwave antennas mounted on transceiver base stations110B, 110D, and 110E. Baseband data traffic being sent from remote RFmodem shelves 140B, 140C, and 140D is transmitted by microwave in thereverse direction (i.e., from transceiver base stations 110B, 110D, and110E to transceiver base stations 110A, 110C, and 110F).

At each of transceiver base stations 110B, 110D, and 110E, downlink datatraffic from central office facilities 160A and 160B is down-convertedfrom microwave frequencies to baseband signals before being up-convertedagain for transmission to subscriber premises within each cell site.Uplink data traffic received from the subscriber premises isdown-converted to baseband signals before being up-converted tomicrowave frequencies for transmission back to central office facilities160A and 160B.

Generally, there is an asymmetry of data usage in the downlink and theuplink. This asymmetry is typically greater than 4:1 (downlink:uplink).Taking into account the factors of data asymmetry, channel propagation,and available spectrum, an advantageous embodiment of the presentdisclosure adopts a flexible approach in which the physical (PHY) layerand the media access (MAC) layer are based on the use of time divisionduplex (TDD) time division multiple access (TDMA). TDD operations sharea single RF channel between a transceiver base station and a subscriberpremises and use a series of frames to allocate resources between eachuser uplink and downlink. A great advantage of TDD operation is theability to dynamically allocate the portions of a frame allocatedbetween the downlink and the uplink. This results in an increasedefficiency of operation relative to frequency division duplex (FDD)techniques. TDD operations typically may achieve a forty to sixtypercent advantage in spectral efficiency over FDD operations undertypical conditions. Given the short duration of the transmit and receivetime slots relative to changes in the channel, TDD operations alsopermit open loop power control, switched diversity techniques, andfeedforward and cyclo-stationary equalization techniques that reducesystem cost and increase system throughput.

To aid with periodic functions in the system, TDD frames are groupedinto superframes (approximately 10 to 20 milliseconds). The superframesare further grouped into hyperframes (approximately 250 to 1000milliseconds). This provides a coordinated timing reference tosubscriber integrated access devices in the system. FIG. 3 illustratesan exemplary time division duplex (TDD) time division multiple access(TDMA) framing hierarchy according to one embodiment of the presentdisclosure. At the highest level, the TDD-TDMA framing hierarchycomprises hyperframe 310, which is X milliseconds (msec.) in length(e.g., 250 msec.<X<1000 msec.). Hyperframe 310 comprises N superframes,including exemplary superframes 311-316. Each of superframes 311-316 is20 milliseconds in duration. Superframe 313 is illustrated in greaterdetail.

Superframe 313 comprises ten (10) TDD frames, including exemplary TDDframes 321-324, which are labeled TDD Frame 0, TDD Frame 1, TDD Frame 2,and TDD Frame 9, respectively. In the exemplary embodiment, each TDDframe is 2 milliseconds in duration. A TDD transmission frame is basedon a fixed period of time during which access to the channel iscontrolled by the transceiver base station.

Exemplary TDD frame 321 is illustrated in greater detail. TDD frame 321comprises a downlink portion (i.e., base station to subscribertransmission) and an uplink portion (i.e., subscriber to base stationtransmission). In particular, TDD frame 321 comprises:

Frame header 330—Frame header 330 is a broadcast message thatsynchronizes the start of frame and contains access control informationon how the remainder of TDD frame 321 is configured. The modulationformat of frame header 330 is chosen so that all subscribers in a sectorof the transceiver base station can receive frame header 330. Generally,this means that frame header 330 is transmitted in a very low complexitymodulation format, such as binary phase shift keying (BPSK or 2-BPSK),or perhaps quadrature phase shift keying (QPSK or 4-BPSK).

D downlink slots—The D downlink slots, including exemplary downlinkslots 341-343, contain transceiver base station-to-subscribertransmissions of user traffic and/or control signals. The modulationformat of each slot is optimized for maximum possible data transmissionrates. Downlink slots may be grouped in blocks to form modulation groupsas shown in FIG. 5A. Subscribers who receive data using the samemodulation format (or modulation index) and the same forward errorcorrection (FEC) codes are grouped together in the same modulationgroup. In some embodiment of the present disclosure, two or moremodulation groups may have the same modulation format and FEC codes. Inalternate embodiments of the present disclosure, downlink slots may begrouped in blocks based on physical beam forming, rather than onmodulation format and FEC codes. For example, a transceiver base stationmay transmit data to several subscribers that are directionally alongthe same antenna beam in consecutive bursts. In still other embodimentsof the present disclosure, downlink slots may be grouped in blocks basedon any combination of two or more of: 1) physical beam forming, 2)modulation format, and 3) FEC codes. For the purpose of simplicity, theterm “modulation group” shall be used hereafter to refer to a group ofdownlink slots that are transmitted to one or more subscribers using acommon scheme consisting of one or more of modulation format, FEC codes,and physical beam forming.

U uplink slots—The U uplink slots, including exemplary uplink slots361-363, contain subscriber-to-transceiver base station transmissions ofuser traffic and/or control signals. Again, the modulation format(modulation index) is optimized for maximum possible data transmissionrates. Generally, the modulation format and FEC codes in the uplinkslots are less complex than in the downlink slots. This moves complexityto the receivers in the base stations and lowers the cost and complexityof the subscriber access device. Uplink slots may be grouped in blocksto form sub-burst groups as shown in FIGURE SA. Subscribers who transmitdata using the same modulation format (or modulation index) and the sameforward error correction (FEC) codes are grouped together in the samesub-burst group. In some embodiments of the present disclosure, two ormore sub-burst groups may have the same modulation format and FEC codes.In other embodiments of the present disclosure, uplink slots may begrouped in blocks based on physical beam forming, rather than onmodulation format and FEC codes. In other embodiments, uplink slots maybe grouped in blocks based on any combination of two or more of: 1)physical beam forming, 2) modulation format, and 3) FEC codes. For thepurpose of simplicity, the term “sub-burst group” shall be usedhereafter to refer to a group of uplink slots that are transmitted toone or more subscribers using a common scheme consisting of one or moreof modulation format, FEC codes, and physical beam forming.

Contention slots 360—Contention slots 360 precede the U uplink slots andcomprise a small number of subscriber-to-base transmissions that handleinitial requests for service. A fixed format length and a singlemodulation format suitable for all subscriber access devices are usedduring contention slots 360. Generally, this means that contention slots360 are transmitted in a very low complexity modulation format, such asbinary phase shift keying (BPSK or 2-BPSK), or perhaps quadrature phaseshift keying (QPSK or 4-BPSK). Collisions (more than one user on a timeslot) result in the use of back-off procedures similar to CSMA/CD(Ethernet) in order to reschedule a request.

TDD transition period 350—TDD transition period 350 separates the uplinkportion and the downlink portion and allows for transmitter (TX) toreceiver (RX) propagation delays for the maximum range of the cell linkand for delay associated with switching hardware operations from TX toRX or from RX to TX. The position of TDD transition period 350 may beadjusted, thereby modifying the relative sizes of the uplink portion andthe downlink portion to accommodate the asymmetry between data trafficin the uplink and the downlink.

Exemplary downlink slot 342 is shown in greater detail. Downlink slot342 comprises burst header 371, encapsulated packet data unit (PDU) 372,and forward error correction check sum value 373. The length of downlinkslot 342 varies according to the modulation format used communicate withthe subscriber access device to which downlink slot 342 is transmitted.The other downlink slots and uplink slots in TDD frame 323 are similarin structure to downlink slot 342.

A key aspect of the present disclosure is that the timing of thedownlink and uplink portions of each TDD frame must be precisely alignedin order to avoid interference between sectors within the same celland/or to avoid interference between cells. It is recalled from abovethat each sector of a cell site is served by an individual RF modem inRF modem shelves 140A-140D and the internal RF modem shelves of centraloffice facilities 160A and 160B. Each RF modem uses an individualantenna to transmit and to receive in its assigned sector. The antennasfor different sectors in the same cell site are mounted on the sametower and are located only a few feet apart. If one RF modem (andantenna) are transmitting in the downlink while another R? modem (andantenna) are receiving in the uplink, the power of the downlinktransmission will overwhelm the downlink receiver.

Thus, to prevent interference between antennas in different sectors ofthe same cell site, an embodiment of the present disclosure may use ahighly accurate distributed timing architecture to align the startpoints of the downlink transmissions. An embodiment of the presentdisclosure may also determine the length of the longest downlinktransmission and ensure that none of the uplink transmissions begin, andnone of the base station receivers begin to receive, until after thelongest downlink is completed.

Furthermore, the above-described interference between uplink anddownlink portions of TDD frames can also occur between different cellsites. To prevent interference between antennas in different cell sites,an embodiment of the present disclosure may also use the highly accuratedistributed timing architecture to align the start points of thedownlink transmissions between cell sites. An embodiment of the presentdisclosure may also determine the length of the longest downlinktransmission among two or more cell sites and ensure that none of thebase station receivers in any of the cells begins to receive in theuplink until after the longest downlink transmission is completed.

Within a cell site, a master interface control processor (ICP), asdescribed below in FIG. 4, may be used to align and allocate the uplinkand downlink portions of the TDD frames for all of the RF modems in anRF modem shelf. Between cell sites, the access processor may communicatewith several master ICPs to determine the longest downlink. The accessprocessor may then allocated the uplinks and downlinks across severalcell sites in order to minimize interference between cell sites and maydesignate on master ICP to control the timing of all of the master ICPs.

FIG. 4 illustrates the timing recovery and distribution circuitry inexemplary RF modem shelf 140 according to one embodiment of the presentdisclosure. RF modem shelf 140 comprises front panel interface 410having connectors 411-414 for receiving input clock references andtransmitting clock references. Exemplary connector 411 receives a firstclock signal from a first external source (External Source A) andexemplary connector 414 receives a second clock signal from a secondexternal source (External Source B). Connector 412 outputs an internallygenerated clock signal (Master Source Out) and connector 413 receives anexternal one second system clock signal (External 1 Second Clock).

RF modem shelf 140 also comprises a plurality of interface controlprocessor (ICP) cards, including exemplary ICP cards 450, 460, 470 and480. ISP card 450 is designated as a master ICP card and ICP card 480 isdesignated as a spare ICP card in case of a failure of master ICP card450. Within RF modem shelf 140, the ICP cards provide for controlfunctions, timing recovery and distribution, network interface, backhaulnetwork interface, protocol conversion, resource queue management, and aproxy manager for EMS for the shelf. The ICP cards are based on networkprocessor(s) that allow software upgrade of network interface protocols.The ICP cards may be reused for control and routing functions andprovide both timing and critical TDD coordinated burst timing for allthe RF modems in RF modem shelf 140 and for shelf-to-shelf timing forstacked frequency high density cell configurations.

The timing and distribution architecture in RF modem shelf 140 allowsfor three reference options:

Primary—An external input derived from another remote modem shelf actingas a master. BITS (Building Integrated Timing Supply) reference is asingle building master timing reference (e.g., External Source A,External Source B) that supplies DS1 and DSO level timing throughout anoffice (e.g., 64K or 1.544/2.048 Mbps).

Secondary—A secondary reference may be derived from any designated inputport in RF modem shelf 140. For remote RF modem shelf 140, this is oneof the backhaul I/O ports. An ICP card is configured to recover a timingsource and that source is placed on a backplane as a reference (i.e.,Network Reference (A/B)) to master ICP card 450.

Tertiary—An internal phase locked loop (PLL) may be used.

By default, two ICP cards are configured as a master ICP card and aspare ICP card. The active master ICP card distributes timing for all ofRF modem shelf 140. The timing distribution architecture of RF modemshelf 140 meets Stratum 3 levels of performance, namely a free-runaccuracy of +/−4.6 PPM (parts per million), a pull-in capability of 4.6PPM, and a holdover stability of less than 255 slips during the firstday.

There are three components to the timing distribution for the accessprocessor backplane:

1. Timing masters (ICP cards 450 and 480).

2. Timing slaves (ICP cards 460 and 470).

3. Timing references.

The timing masters are capable of sourcing all clocks and framingsignals necessary for the remaining cards within the AP backplane.Within a backplane, there are two timing masters (ICP cards 450 and480), which are constrained to the slots allocated as the primary andsecondary controllers. The timing masters utilize the redundant timingreferences (External Source A, External Source B, External 1 SecondClock) found on the backplane to maintain network-qualifiedsynchronization. ISP card 450 (and ISP card 480) comprises backhaulnetwork input/output (I/O) port 451, multiplexer 452 and PLL-clockgenerator 453. MUX 452 selects anyone of External Source A, ExternalSource B, Network Reference (A/B), and the signal from I/O port 451 tobe applied to PLL-clock generator 453. The timing master has missingclock detection logic that allows it to switch from one timing referenceto another in the event of a failure.

Timing is distributed across a redundant set of clock and framingsignals, designated Master Clock Bus in FIG. 4. Each timing master(i.e., ICP cards 450 and 480) is capable under software control ofdriving either of the two sets of clock and framing buses on thebackplane. Both sets of timing buses are edge-synchronous such thattiming slaves can interoperate while using either set of clocks.

The timing supplied by the timing master (e.g., ICP card 450) consistsof a 65.536 MHZ clock and an 8 KHz framing reference. There is a primaryand secondary version of each reference. To generate these references,the primary and secondary timing masters are provisioned to recover thetiming from one of the following sources:

Table of Clock Source Interface Definitions Source Connector FrequencyExternal BITS 75/120 Ohm BNC 64K, 1544K, 2048K (EXT REF A) ExternalBITS/GPS 75/120 Ohm, DB9 64K, 1544K, 2048K (EXT REF B) External GPS Sync75/120 Ohm, DB9 1 sec pulse Pulse On card Reference Digital Logic LevelPer I/O reference Network I/O derived Digital Logic Level Per I/Oreference Reference A Network I/O derived Digital Logic Level Per I/Oreference Reference B

To simplify clock distribution and to provide redundancy all the clocksare derived from a common clock source. The following table summarizesthe backplane reference clocks as well as the clock rates of the variousbackplane resources and how they are derived from these references.

Table of Buses and Associated Clocks Clock Frequency Division or RatioCommon Reference 65.536 MHZ Not Applicable Clock Common Sync Pulse 1 HzNot Applicable Framing Reference 8 KHz Free-run framing (125 usec)provided by Primary or Secondary Clock Masters Referenced to CommonReference Clock Cell/Packet Clock 32.768 MHZ Reference Clock/2 Rate TDMBus Rate 8.192 MHZ Reference Clock/8 RF Reference Clock 10.000 MHZFree-run RF reference clock Communications Bus 100 MHZ Derived fromfree- run Bus Reference Clock High-speed Serial 1.31072 GHz Ref Clock X20 Links

Timing slaves (i.e., ICP cards 460 and 470) receive the timing providedby redundant sets of clock and framing buses. Under software control,timing slaves choose a default set of clocks from either the A-side orB-side timing buses. They also contain failure detection logic such thatclock and framing signal failures can be detected. Once a clock orframing failure is detected, the timing slave automatically switches tothe alternate set of timing buses. ICP cards 460 and 470 containbackhaul I/O ports 461 and 471, respectively, which may be used to bringin external timing signals from other RF modem shelves in the network.The timing masters (i.e., ICP cards 450 and 480) also contain the timingslave function insofar as they also utilize the timing provided on thebackplane clock and framing buses.

A qualified timing reference is required for the timing master to derivebackplane timing and to maintain synchronization within network 100 andwith any outside network. Under software control, an access processorcard can be assigned to derive this timing and to drive one of the twotiming reference buses. Ideally, a second, physically separate card willcontain a second qualified timing source and drive the second backplanetiming reference.

In the event that no qualified timing is present from trunk interfaces,the access processor backplane has connections which allow externalreference timing (e.g., a GPS-derived clock) from the interface tray tobe applied to the backplane. A one pulse-per-second (1PPS) signal isdistributed to all system cards for time stamping of system events anderrors. Installations involving multiple access processor shelvesrequire the timing reference to be distributed between all accessprocessor backplanes. In this scenario, the timing reference for a givenbackplane is cabled to the remaining backplanes through externalcabling. Multiple remote modem shelves are utilized to distributehigh-capacity backhaul traffic to one or more additional co-locatedmodem shelves. Traffic is distributed among the shelves through T1, T3,OC3 and/or other broadband telecommunication circuits. To maintainnetwork timing, the additional shelves are slaved to these distributionlinks and recover timing through the same PLL mechanisms as the head-endshelf.

FIG. 5 illustrates exemplary interface tray 1500 associated with RFmodem shelf 140 according to one embodiment of the present disclosure.Interface tray 1500 comprises signal conditioning-10 MHz oscillatorcircuitry 1505, alarm conditioning circuitry 1510, RF circulator-powerdivider circuitry 1515, and 6:1 switches 1520 and 1525. Exemplaryinterface tray 1500, located above remote modem shelf 140, is thejunction at which the cell site antennas and the RF modems interconnect.Interface tray 1500 provides N+1 redundancy among the RF modems in RFmodem shelf 140, using an RF distribution circuit housed withininterface tray 1500. In addition to the antenna feeds, all externalalarms, the BITS and GPS timing signals, control signals, and powersupplies (not shown) are interfaced through interface tray 1500. Accessprocessor shelf 165 shares the same interface tray design.

All access to the cell tower antennas, alarms, power, I²C, and BITStiming and GPS signals are accomplished through rear panel 1501 ofinterface tray 1500. RF signals supplied to the RF modem cards arereceived through front panel 1502 of the tray. All communications andcontrol with interface tray 1500 are done via discrete connections.Control functions with interface tray 1500 via the remote modem ICPcards are:

1. Switching of antennas to the redundant RF Modem

2. Alarm indications from external alarms

3. CO Output Alarm Indication

Any external alarms that are detected are conditioned as necessary byalarm conditioning circuitry 1510 for output to the primary andsecondary master ICP cards in remote RF modem shelf 140 via the discreteinterconnections. For CO alarm requirements, the system will output analarm to the facility switching equipment via relay contact closure.

Interface tray 1500 serves three timing input sources, namely the BITSsignal, the GPS signal, and the GPS 1 PPS signal. These timing signalsare conditioned by signal conditioning-10C MHz oscillator circuitry1505, as required, before being transmitted out front panel 1502 forinterfacing to RF modem shelf 140. Interface tray 1500 supportsdiversity reception required by the RF modems. One channel of thediversity pair is dedicated to transmission. That channel is fed by oneof the RF circulators in RF circulator-power divider circuitry 1515 toallow for transmission and reception and to support redundantswitchover. The second channel is a receive-only channel. One of the RFpower dividers in RF circulator-power divider circuitry 1515 feeds thereceive only channel.

To provide N+1 redundancy in the remote modem shelf 140, a switchoverscheme must be devised. For the purposes of discussion, a six sectorcell site is assumed. In this scheme, both RP feeds for each RF modemchannel must be fed to one of 6:1 switches 1520 and 1525. Switching ischosen over power division to reduce the path loss through the channelversus a power division scheme. All of the TX/RX signals are fed to 6:1switch 1520 and all of the RX only signals are to 6:1 switch 1525. Upondetection of an RF modem failure, master ICP card 450 is notified andthe spare modem is switched in.

There is a stable 10 MHz oscillator circuit in signal conditioning-10MHz oscillator circuitry 1505 in interface tray 1500. The 10 MHz signalis used to phase reference all of the RF modem cards. A low-cost backuposcillator is available in interface tray 1500 in the event of failureof the primary oscillator. The backup oscillator is phased locked withthe GPS signal to allow for enough stability to operate untilmaintenance can be performed on interface tray 1500.

FIG. 5A illustrates exemplary time division duplex (TDD) frame 500according to one embodiment of the present disclosure. FIG. 5Billustrates exemplary transmission burst 520 containing a single FECblock according to one embodiment of the present disclosure. FIG. 5Cillustrates exemplary transmission burst 530 containing multiple FECblocks according to one embodiment of the present disclosure.

TDD frame 500 comprises a downlink portion containing preamble field501, management field 502, and N modulation groups, including modulationgroup 503 (labeled Modulation Group 1), modulation group 504 (labeledModulation Group 2), and modulation group 505 (labeled Modulation GroupN). As explained above in FIG. 3, a modulation group is a group ofdownlink slots transmitted to one or more subscribers using a commonscheme of one or more of; 1) modulation format, 2) FEC codes, and 3)physical beam forming.

TDD frame 500 also comprises an uplink portion containingtransmitter-transmitter guard (TTG) slot 506, 0 to N registration (REG)minislots 506, 1 to N contention (CON) request minislots 508, Nsub-burst groups, including sub-burst group 509 (labeled Sub-Burst 1)and sub-burst group 510 (labeled Sub-Burst N), and receiver-transmitterguard (RTG) slot 511. As explained above in FIG. 3, a sub-burst group isa group of uplink slots transmitted to one or more subscribers using acommon scheme of one or more of: 1) modulation format, 2) FEC codes, and3) physical beam forming.

Each modulation group and each sub-burst group comprises one or moretransmission bursts. Exemplary transmission burst 520 may be used withina single modulation group in the downlink and covers one or moredownlink slots. Transmission burst 520 also may be used within a singlesub-burst group in the downlink and covers one or more uplink slots.Transmission burst 520 comprises physical media dependent (PMD) preamblefield 521, MAC header field 522, data packet data unit (PDU) field 523,and block character redundancy check (CRC) field 524. Transmission burst530 comprises physical media dependent (PMD) preamble field 531, MACheader field 532, data PDU field 533, block CRC field 534, data PDUfield 535, block CRC field 536.

The start of every frame includes a Start-Of-Frame (SOF) field and a PHYMedia Dependent Convergence (PMD) field. PMD preambles are used toassist in synchronization and time-frequency recovery at the receiver.The SOF field allows subscribers using fixed diversity to test receptionconditions of the two diversity antennas.

The SOF PMD field is 2^(N) symbols long (typically 16, 32, 64 symbolslong) and consists of pseudo-random noise (PN) code sequences, Franksequences, CAZAC sequences, or other low cross-correlation sequences,that are transmitted using BPSK or QPSK modulation. The SOF field isfollowed by downlink management messages broadcast from the base stationto all subscribers using the lowest modulation or FEC index andorthogonal expansion. Management messages are transmitted bothperiodically (N times per hyperframe) and as required to changeparameters or allocate parameters. Management messages include:

-   -   1. DownLink Map indicating the physical slot (PS) where        downstream modulation changes (transmitted every frame);    -   2. UpLink MAP indicating uplink subscriber access grants and        associated physical slot start of the grant (transmitted when        changed and at a minimum of one second hyperframe periods        (shorter periods are optional));    -   3. TDD frame and physical layer attributes (periodic at a        minimum of one second hyperframe period); and    -   4. Other management messages including ACK, NACK, ARQ requests,        and the like (transmitted as required).

The downlink management messages are followed by multi-cast and uni-castbursts arranged in increasing modulation complexity order. The presentdisclosure introduces the term “modulation group” to define a set ofdownstream bursts with the same modulation and PEC protection. Asubscriber continuously receives all the downstream data in the TDDframe downlink until the last symbol of the highest modulation groupsupported by the link is received. This allows a subscriber maximum timeto perform receive demodulation updates.

The downlink-to-uplink transition provides a guard time (TTG) to allowfor propagation delays for all the subscribers. The TTG position andduration is fully programmable and set by management physical layerattribute messages. The TTG is followed by a set of allocated contentionslots that are subdivided between acquisition uplink ranging mini-slotsand demand access request mini-slots. The Uplink MAP message establishesthe number and location of each type of slot. Ranging slots are used forboth initial uplink synchronization of subscribers performing net entryand for periodic update of synchronization of active subscribers.Contention slots provide a demand access request mechanism to establishsubscriber service for a single traffic service flow. As collisions arepossible, the subscriber uses random back-off, in integer TDD frameperiods and retries based on a time out for request of service.Contention slots use the lowest possible modulation, FEC, and orthogonalexpansion supported by the base station.

The contention slots are followed by individual subscriber transmissions(sub-bursts) that have been scheduled and allocated by the base stationin the uplink MAP. Each subscriber transmission burst is performed atthe maximum modulation, FEC, and orthogonal expansion supported by thesubscriber. Finally, the subscriber transmissions are followed by theuplink-to-downlink transition which provides a guard time (RTG) to allowfor propagation delays for all the subscribers. The RTG duration isfully programmable and set by management physical layer attributemessages.

In the downlink, the Physical Media Dependent (PMD) burstsynchronization is not used. The transmission burst begins with the MACheader and is followed by the packet data unit (PDU) and the associatedblock CRC field that protects both the PDU and the header. The PDU maybe a complete packet transmission or a fragment of a much largermessage. When a channel requires more robust FEC, the PDU may be brokeninto segments that are protected by separate FEC CRC fields. This avoidswasting bandwidth with additional MAC headers.

One significant difference between the uplink and the downlink is theaddition of the PMD preamble. The PMD preamble length and pattern can beprogrammed by transceiver base station 110. Like the SOF field at thebeginning of the TDD Frame, the preamble provides a synchronizationmethod for the base station receiver. Uplink registration and rangingpacket bursts use longer PMD preambles.

The medium access control (MAC) layer protocol is connection orientedand provides multiple connections of different quality of service (QoS)to each subscriber. The connections are established when a subscriber isinstalled and enters operation fixed wireless access network 100.Additional connections can be established and terminated with the basestation transceivers as subscriber requirements changes.

As an example, suppose a subscriber access device supports two voicechannels and a data channel. The quality of service (QoS) on both of thevoice channels and data can set based on the service structure set bythe wireless service provider. At installation, a subscriber may startwith two service connections: a toll quality voice channel and a mediumdata rate broadband data connection. At a later point in time, thesubscriber may order and upgrade service to two toll quality voicechannels and high speed data connection (a total of three connections).

The maintenance of connections varies based on the type of connectionestablished. T1 or fractional T1 service requires almost no maintenancedue to the periodic nature of transmissions. A TCP/IP connection oftenexperiences bursty on-demand communication that may be idle for longperiods of time. During those idle periods, periodic ranging andsynchronization of the subscriber is required.

In an exemplary embodiment of fixed wireless access network 100, eachsubscriber maintains a 64-bit EUI for globally unique addressingpurposes. This address uniquely defines the subscriber from within theset of all possible vendors and equipment types. This address is usedduring the registration process to establish the appropriate connectionsfor a subscriber. It is also used as part of the authentication processby which the transceiver base station and the subscriber each verify theidentity of the other.

In the exemplary embodiment, a connection may be identified by a 16-bitconnection identifier (CID) in MAC header 522 or MAC header 532. Everysubscriber must establish at least two connections in each direction(upstream and downstream) to enable communication with the base station.The basic CIDs, assigned to a subscriber at registration, are used bythe base station MAC layer and the subscriber MAC layer to exchange MACcontrol messages, provisioning and management information.

The connection ID can be considered a connection identifier even fornominally connectionless traffic like IP, since it serves as a pointerto destination and context information. The use of a 16-bit CID permitsa total of 64K connections within the sector.

In an exemplary embodiment of fixed wireless access network 100, the CIDmay be divided into 2 fields. Bits [16:x] may be used to uniquelyidentify a subscriber. In a cyclo-stationary receiver processing at abase station, this would set the antenna, equalizer, and other receiverparameters. Bits [x:1] may be used to indicate a connection to a type ofservice. Each subscriber service can have individual modulation format,FEC, and ARQ. Thus, within a single sub-burst group transmitted by asubscriber, the voice data may use one type of modulation format, FEC,and ARQ, and the broadband internet service may use a differentmodulation format, FEC, and ARQ. Similarly, within a single modulationgroup transmitted to the subscriber, the voice data may use one type ofmodulation format, FEC, and ARQ, and the broadband internet service mayuse a different modulation format, FEC, and ARQ.

As an example, bits [16:7] of the CID may identify 2{circumflex over( )}10 (or 1024) distinct subscribers and bits [6:1] may identify2{circumflex over ( )}=64 possible connections. An apartment buildingcould be given a set of subscriber ports [16:9] so that bits [9:7] allow2{circumflex over ( )}8 connections or 256 connections.

Requests for transmission are based on these connection IDs, since theallowable bandwidth may differ for different connections, even withinthe same service type. For example, a subscriber unit serving multipletenants in an office building would make requests on behalf of all ofthem, though the contractual service limits and other connectionparameters may be different for each of them.

Many higher-layer sessions may operate over the same wireless connectionID. For example, many users within a company may be communicating withTCP/IP to different destinations, but since they all operate within thesame overall service parameters, all of their traffic is pooled forrequest/grant purposes. Since the original LAN source and destinationaddresses are encapsulated in the payload portion of the transmission,there is no problem in identifying different user sessions.

Fragmentation is the process by which a portion of a subscriber payload(uplink or downlink) is divided into two or more PDUs. Fragmentationallows efficient use of available bandwidth while maintaining the QoSrequirements of one or more of services used by the subscriber.Fragmentation may be initiated by a base station for a downlinkconnection or the subscriber access device for the uplink connection. Aconnection may be in only one fragmentation state at any given time. Theauthority to fragment data traffic on a connection is defined when theconnection is created.

The MAC layer protocol in wireless access network 100 also supportsconcatenation of multiple PDUs in a single transmission in both theuplink and the downlink, as shown in FIG. 5C. Since each PDU contains aMAC header with the CID, the receiving MAC layer can determine routingand processing by higher layer protocols. A base station MAC layercreates concatenated PDUs in the uplink MAP. Management, traffic data,and bandwidth may all be concatenated. This process occurs naturally inthe downlink. In the uplink, concatenation has the added benefit ofeliminating additional PMD preambles.

FIG. 6 depicts flow diagram 600, which illustrates the adaptivemodification of the uplink and downlink bandwidth in the air interfacein wireless access network 100 according to one embodiment of thepresent disclosure. Initially, an RF modem shelf, such as RF modem shelf140A, receives new access requests from subscriber access devices infixed wireless access network 100 and determines traffic requirementsfor each new and existing subscriber in each sector of a single cellsite (process step 605). The traffic requirements of each subscriber maybe established in a number of ways, including minimum QoS requirements,service level agreements, past usage, and current physical layerparameters, such as modulation index, FEC codes, antenna beam forming,and the like. The RF modem shelf then determines from the subscribertraffic requirements the longest downlink portion of any TDD frame ineach sector of a single cell site (process step 610).

Next, the access processor for the RF modem shelf (or the RF modem shelfitself) determines the appropriate allocation of downlink and uplinkportions of TDD frames for a single cell site in order to minimize oreliminate interference within the cell site (process step 615).Bandwidth is allocated, and TDD transition period 350 is positioned,such that the longest downlink transmission is complete before anyreceiver in the cell site starts to listen for the uplink transmission.

Next, if global allocation of downlink and uplink bandwidth acrossmultiple cell sites is being implemented (generally the case), theaccess processor determines the longest downlink portion of any TDDframe across several closely located cell sites stations (process step620). The access processor then determines the allocation of uplink anddownlink bandwidth for all TDD frames across several closely locatedcell sites in order to minimize or eliminate cell-to-cell interference(process step 625). Again, bandwidth is allocated, and TDD transitionperiod 350 is positioned, such that the longest downlink transmission iscomplete before any receiver in any of the closely located cell sitesstarts to listen for uplink transmissions. Finally, the downlinkportions of the TDD frames are launched simultaneously using the highlyaccurate clock from the distributed timing architecture (process step630).

The dynamic application of TDD bandwidth allocation is bounded by setminimum and maximum boundaries set by the service provider, based ontraffic and network analysis. Further, the bandwidth bounds may beallocated in sub-groupings based on established quality of service (QoS)requirements (e.g., voice data) and Service Level Agreements (SLA)(e.g., broadband data rate) as the primary consideration and with bestefforts, non-QoS data, and IP traffic as secondary considerations. Thebandwidth bounds may be allocated based on the fact that a subscribermay support more that one interface and thus more than one modulationformat in order to achieve required error rates for one or more servicesprovided to the subscriber.

FIG. 7 depicts flow diagram 700, which illustrates the adaptiveassignment of selected link parameters, such as modulation format,forward error correction (FEC) codes, and antenna beam forming, to theuplink and downlink channels used by each subscriber in wireless accessnetwork 100 according to one embodiment of the present disclosure. TheRF modem shelf monitors data traffic between subscribers and basestation and determines for each subscriber the most efficientcombination of modulation format, FEC code, and/or antenna beam formingfor the uplink and downlink.

The selected combination is based at least in part on the error ratesdetected by the RF modem shelf when monitoring the data traffic. If theerror rate for a particular subscriber is too high in either the uplinkor the downlink, the RF modem shelf can decrease the modulation formatcomplexity and use a higher level of FEC code protection in either theuplink or the downlink in order to reduce the error rate. Conversely, ifthe error rate for a particular subscriber is very low in either theuplink or the downlink, the RF modem shelf can increase the modulationformat complexity and use a lower level of FEC code protection in eitherthe uplink or the downlink in order to increase the spectral efficiency,provided the error rate remains acceptably low. Different modulationformats and FEC codes may be used for different services (e.g., voice,data) used by a subscriber (process step 705).

Next, the RF modem shelf assigns subscribers to modulation groups in thedownlink and to sub-burst groups in the uplink (process step 710). Thebase station transceiver then transmits media access fields (e.g.,signaling, ACK & NACK) using the lowest modulation format/FEC codecomplexity. The base station transceiver then transmits the remainingmodulation groups in the downlink to the subscribers in increasing orderof modulation format/FEC code complexity (process step 715). When thedownlink is complete, the base station transceiver receives registration& contention minislots transmitted by the subscriber access devicesusing the lowest modulation format/FEC code complexity. The base stationtransceiver then receives the remaining sub-burst groups transmitted bythe subscribers in increasing order of modulation format/FEC codecomplexity (process step 720).

The use of adaptive link parameters improves the link throughput andcorrespondingly affects the bandwidth allocation described above in FIG.6. Link parameters apply not only to the transmitter but to the receiveras well. Some embodiments of the present disclosure may use a bounded(finite) set of modulation formats to maximize bandwidth utilization toeach subscriber in a channel or sector. In an exemplary embodiment ofthe present disclosure, the low complexity (low bandwidth efficiency)modulation formats used for media access fields (e.g., signaling, ACK,NACK) are binary phase shift keying (BSPK or 2-PSK) and quadrature phaseshift keying (QPSK or 4-PSK). Some embodiments of the present disclosuremay also use multiple-code orthogonal expansion codes in conjunctionwith the low complexity modulation formats for extremely robustcommunication. The higher complexity (higher efficiency) modulationformats used for the modulation groups and the sub-burst groups may be8-PSK, 16 quadrature amplitude modulation (QAM), 32 QAM, 64 QAM, 128QAM, and the like.

Some embodiments of the present disclosure may also use a bounded set ofFEC codes to maximize bandwidth utilization to each subscriber in achannel or sector. The level of FEC code protection is based on theservices provided. Each subscriber may support multiple services.

In an advantageous embodiment of the present disclosure, the RF modemshelf may use packet fragmentation to transport data in either theuplink or the downlink. Fragmentation is the division of larger packetsinto smaller packets (fragments) combined with an ARQ (automatic requestfor retransmission) mechanism to retransmit and recover erroneousfragments. The RF modem shelf automatically reduces fragment size forhigh error rate channels. Fragmentation is applied for guaranteederror-free sources. The degree of fragmentation and ARQ is based on theservice provided, since each subscriber may support multiple services.

FIG. 8 depicts flow diagram 800, which illustrates the adaptiveassignment of selected link parameters to the different serviceconnections used by each subscriber in wireless access network 100according to one embodiment of the present disclosure. The RF modemshelf assigns connection identification (CID) values to the uplink andto the uplink connections used by a subscriber. If a subscriber usesmore than one service (e.g., two voice, one data), the RF modem shelfassigns separate CID values to each uplink connection and separate CIDvalues to each downlink connection (process step 805). As noted above,the CID comprises a bit field with the uppermost bits identifying thesubscriber and the lowermost bits identifying a specific connection tothe subscriber. While many sets of adaptive transmission and receptionparameters are possible, there are a finite number of combinations thatmake logical sense. These combinations are grouped into physical layerusage codes that are broadcast to subscribers as part of the generalheader of TDD superframe or frame header on a periodic basis. Theseapply to both the base station transmissions and the subscribertransmissions.

The RF modem shelf monitors data traffic between subscriber and basestation and determines for each connection the most efficientcombination of modulation format, FEC code, and/or antenna beam formingfor the uplink and downlink (process step 810). The RF modem shelf thenassigns each subscriber connection to a modulation group in the downlinkand to a sub-burst group in the uplink (process step 815). The basestation transmits media access fields (e.g., signaling, ACK & NACK)using the lowest modulation format/FEC code complexity. Then basestation then transmits modulation groups to subscribers in increasingorder of modulation format/FEC code complexity (process step 820).Finally, the base station receives registration & contention minislotsusing the lowest modulation format/FEC code complexity. Then basestation then receives sub-burst groups from subscribers in increasingorder of modulation format/FEC code complexity (process step 825).

Physical layer usage codes are bound to subscriber CID values by aservice establishment protocol. If there is a degradation or improvementin the channel between a subscriber and the base station, a protocolexists so the subscriber access device and the base station may revisethe physical layer usage code and subscriber CID code. The codes andbindings can be added and deleted based on services requirements of thesubscriber.

FIG. 9A depicts a cell and sector layout for a wireless access coveragearea according to one embodiment of the present disclosure. Coveragearea 900 is logically divided into cells 910, 920, 930 and 940 eachlogically divided into a number of sectors 911-916, 921-926, 931-936 and941-946, respectively. Each cell 910, 920, 930 and 940 includes atransceiver base station 110 as depicted in FIG. 1 at a central location917, 927, 937, and 947, respectively, as well as subscriber premises121-123 within the coverage area of the respective cell.

Sectors 911-916, 921-926, 931-936 and 941-946 are logically divided intotwo categories: those designated sector type “A” and those designatedsector type “B”, with sector categories alternating within a cell sothat no two adjacent cells fall in the same category and with cellsarranged so that no two adjacent sectors from adjoining cells fall inthe same category. Each sector is falls within a different category thanall other adjacent sectors with which the respective sector shares acommon linear boundary.

FIGS. 10C through 10E are high level timing diagrams illustratingbandwidth allocation among sectors according to one embodiment of thepresent disclosure, and are intended to be read in conjunction with FIG.9A. An embodiment of the present disclosure may incorporate FDDoperation, with dedicated downlink and uplink channels, within a TDDsystem by introducing a frequency change at the normal TDD guard point.Transmission time on the dedicated downlink frequency F1 and thededicated uplink frequency F2 are divided between adjacent sectorswithin categories A and B. Thus, the TDD FDD system 1020 of theembodiment of the present disclosure shown in FIG. 10C allocates both adownlink period 1021, 1022 on the downlink frequency F1 and an uplinkperiod 1023, 1024 on the uplink frequency F2 to each of the sectorswithin categories A and B.

The allocated periods 1012/1022 and 1023/1024 are offset in both timeand frequency, then overlaid so that the sector A downlink period 1021does not coincide in time or frequency with the sector A uplink period1024 and the sector B downlink period 1022 does not coincide in time orfrequency with sector B uplink period 1023. Instead, downlinktransmission 1021 in each sector within category A occurs at the sametime as uplink transmission 1023 within each sector within category B,while downlink transmission 1022 in each sector within category B occursconcurrently with uplink transmission 1024 for each sector withincategory A.

In this manner, the dedicated downlink frequency F1 and the dedicateduplink frequency F2 are time-shared by adjacent sectors, but remaindedicated to downlink or uplink transmission and may utilize FDD-onlybandwidth within the MMDS spectrum. Duplex spacing 1013 between downlinkand uplink frequencies F1 and F2 (typically 50-70 MHz) is alsomaintained.

FIG. 11 depicts in greater detail a frame structure employed within theexemplary bandwidth allocation scheme according to one embodiment of thepresent disclosure, and is intended to be read in conjunction with FIGS.9 and 10C through 10E. The frame 1100 depicted corresponds to each ofthe sectors within category A described above and depicted in FIGS. 9Aand 10C through 10E, although each sector within category would utilizea similar frame, as described in further detail below.

Frame 1100 includes a frame header 1110, an downlink sub-frame 1120, andan uplink sub-frame 1130, with the downlink and uplink sub-frameslogically divided into a number of physical slots 1140. The frame header1110 includes a preamble 1111 containing a start-of-frame field, whichallows subscribers using fixed diversity to test reception conditions ofthe two diversity antennas, and a physical layer (the air interface islayered as a physical layer and a media access layer) media dependentconvergence field, utilized to assist in synchronization andtime/frequency recovery at the receiver. The preamble 1111 is followedby media access management information 1112, which includes a downlinkMAP identifying the physical slot where the downlink ends and the uplinkbegins, an uplink MAP indicating uplink subscriber access grants and theassociated physical slot start of the grant, and other managementmessages such as acknowledge (ACK) response, etc.

During the downlink sub-frame 1120, the base transmitter and thesubscriber receiver are both set to the downlink frequency F1. Thedownlink sub-frame 1120 terminates with a frequency change physical slot1121, during which multi-stage digital filters within both the base andthe subscriber unit are altered to switch to the uplink frequency F2,followed by a transmitter transition guard time 1122, during which notransmission occurs to allow for propagation delays for all subscriberunits. The transmitter transition guard time 1122, depicted as occupyingthree physical slots in FIG. 11, is fully programmable both in positionand duration, set by management physical layer attribute messages.

During the downlink sub-frame 1130, the base receiver and the subscribertransmitter(s) are both set to the uplink frequency F2. The firstphysical slots within the uplink sub-frame 1130 are subscriberregistration or acquisition uplink ranging slots, utilized for bothinitial uplink synchronization of subscribers performing entry into thenetwork and periodic update of synchronization of active subscribers,followed by contention slots, providing a demand access requestmechanism to establish subscriber service for a single traffic serviceflow. When collisions occur within the contention slots, the subscriberemploys a random back-off in integer frame periods and retries based ona time out for request of service. Contention slots use the lowestpossible modulation, forward error correction (FEC), and orthogonalexpansion supported by the base. The number and position of registrationand contention slots within the uplink sub-frame 1130 is set by theuplink MAP message within the media access management informationportion 1112 of the frame header 1110.

The contention slots within the uplink sub-frame 1130 are followed byindividual subscriber transmissions which have been scheduled andallocated by the base in the uplink MAP, with each subscribertransmission burst performed at the maximum modulation, FEC andorthogonal expansion supported by the subscriber unit. The uplinksub-frame 1130 terminates with a frequency change physical slot 1131,during which both the base and the subscriber unit switch to thedownlink frequency F1, followed by a receiver transition guard time1132, which is also programmable.

Frames for sectors falling within category B will have a similarstructure, but will be offset so that the downlink sub-frame of eachcategory B sector corresponds in time with the uplink sub-frame of eachcategory A sector, and the uplink sub-frame of each category B sectorcorresponds in time with the downlink sub-frame of each category Asector. The boundary between downlink and uplink sub-frames is adaptiveutilizing block equalization and burst timing coordination. Accordingly,uplink and downlink allocations to sectors in categories A and B may bedivided equally as shown in FIG. 10C, or may be split to allow greatertime within a particular frame to the downlink for sectors in categoryA, as shown in FIG. 10D, or to the downlink for sectors in category B,as shown in FIG. 10E. Spectral efficiency is therefore improved byadapting to the instantaneous traffic requirements among varioussectors.

While the exemplary embodiment is described above with six sector cellsand only two sector categories, the present disclosure may be extendedto any number of sector categories equal to a power of 2 (e.g., 2, 4, 8,. . . , etc.), and preferably employs four sector categories. Where morethan two sector categories are employed, downlink and uplink frequenciesmay be reused in pairs or in staggered offsets (e.g., each sector Ashares a downlink frequency F1 with one adjacent sector B but shares anuplink frequency F2 with a different adjacent sector C, etc.). FIG. 9Bdepicts a cell and sector layout for a wireless access coverage areaaccording to an alternative embodiment of the present disclosure.Coverage area 950 is logically divided into cells 960, 970, 980 and 990each logically divided into four sectors 961-964, 971-974, 981-984 and991-994, respectively. Each cell 960, 970, 980 and 990 includes atransceiver base station 110 as depicted in FIG. 1 at a central location965, 975, 985, and 995, as well as subscriber premises 121-123 withinthe coverage area of the respective cell.

Sectors 961-964, 971-974, 981-984 and 991-994 in the alternativeembodiment are logically divided into four categories, designated sectortype “A”, “B”, “C” and “D”, with sector categories arranged within acell and between cells so that no two adjacent cells fall in the samecategory and no cell adjoins two or more cells in the same category.Each sector falls within a different category than all other adjacentsectors with which the respective sector shares a common linearboundary.

FIG. 12 is functional diagram of filtering employed for wirelesscommunication within each cell in accordance with one embodiment of thepresent disclosure, and is intended to be read in conjunction with FIGS.1, 9A, 93, 10C-10E, and 11. The filtering system 1200 depicted isimplemented within each transceiver base station 110 and each subscriberaccess device on subscriber premises 121-123. The parameters forfiltering system 1200 implemented within each subscriber premises121-123 will be described, although those skilled in the art willrecognize that the filtering systems within each transceiver basestation 110 will simply have the transmission and reception frequencies(i.e., downlink or uplink frequencies F1 and F2) reversed or otherwisechanged.

Wireless signals at the appropriate downlink and uplink frequencies F1and F2 for the subject cell and sector are transmitted and received viaantenna 1201 and separated by a diplexer 1202. Signals received from orpassed to diplexer 1202 are filtered utilizing filters 1203 and 1204tuned to downlink and uplink frequencies F1 and F2, respectively. Thesignal received from filter 1203 is mixed with a signal from a localoscillator 1205 tuned to the downlink frequency F1, while the signaltransmitted to filter 1204 is mixed with a signal from a localoscillator 1206 tuned to the uplink frequency F2. If direct conversionis utilized, the output of mixer 1207 may be connected directly toanalog-to-digital (A/D) converter 1208, and the input to mixer 1209 maybe connected directly to digital-to-analog (D/A) convert 1210.

If super heterodyne conversion is employed, as is preferable, filteringsystem 1200 includes a second (optional) conversion stage 1211. Withinconversion stage 1211, the output of mixer 1207 passes to a filter 1212tuned to an image frequency based on the downlink frequency F1, with thefiltered output being mixed with a signal from a local oscillator 1213also tuned to the image frequency based on downlink frequency F1 beforebeing passed to A/D converter 1208. Similarly, signals from D/Aconverter 1210 are mixed with a signal from a local oscillator 1214tuned to an image frequency based on the uplink frequency F2 and ispassed through a filter 1215 also tuned to the image frequency based onthe uplink frequency F2 before being passed to mixer 1209. A/D and D/Aconverters 1208 and 1210 are coupled to a digital modulator/demodulator1216 which decodes and generates the digital signals from the wirelesscommunications downlinks and uplinks. Additional digital filtering 1217may optionally be employed between A/D converter 1208 andmodulator/demodulator 1216. The filters 1203, 1204, 1212 and 1215,mixers 1207, 1209, 1218 and 1219, A/D/ and D/A converters 1208 and 1210,digital filter 1217, and digital modulator/demodulator 1216 may beimplemented in either hardware or software, collectively, individually,or in any combination of the individual elements.

Filtering system 1200 should have two essential characteristics forsuccessful implementation of a TDD FDD system in accordance with thepresent disclosure. First, the frequency switching time between theuplink and downlink frequencies for the filtering system 1200 within alltransceivers (within each transceiver base station 110 and eachsubscriber premises 121-123) must be sufficiently fast to completeduring the frequency change physical slots 1121 and 1131. Frequencychange physical slots 1121 and 1131, together with guard times 1122 and1132, insure that transmission of an uplink/downlink sub-frame iscompleted successfully before transmission of the next sub-frame isstarted. Frequency switching should preferably take no longer than ¼ to1/10 the duration of physical slots 1121 and 1131. Physical slots 1121and 1131 and/or guard times 1122 and 1132 may alternatively be extendedin duration to accommodate longer frequency switching times within atransceiver between the downlink and uplink frequencies.

Second, filtering system 1200 must filter transmitted and receivedsignals in depth to ensure, in conjunction with the duplex spacingemployed between the downlink and uplink frequencies F1 and F2, thatspurious out-of-band transmission products do not interfere with thereceiver. FIG. 13 illustrates a spectral response for filtering employedfor wireless communication within each cell and sector in accordancewith one embodiment of the present disclosure. A signal strength 1300 atwhich unacceptable interference prevents successful communication may beidentified or defined for a particular system. Filtering system 1200should pass signals within the band 1301 allocated to downlink frequencyF1 and within the band 1302 allocated to uplink frequency F2. By virtueof duplex spacing 1013 between the downlink and uplink frequencies F1and F2, together with the in-depth filtering performed by filteringsystem 1200, out-of-band signals are sufficiently rejected to preventthe signal strength from approaching interference level 1300.

FIG. 14 is functional diagram of filtering employed for wirelesscommunication within each cell and sector in accordance with anotherembodiment of the present disclosure. Filtering system 1400 receiveswireless signals at the appropriate downlink and uplink frequencies F1and F2 for the subject cell and sector via antenna 1201. Signalsreceived from or passed to antenna 1201 are filtered utilizing filter1401, which covers the full FDD band employed for the subject sector. Aswitch 1402 selective connects the filter 1401 to a power amplifier (PA)1403 for transmission or to a low noise amplifier (LNA) 1404 forreception.

In the embodiment depicted in FIG. 14, the conversion stages coupled topower amplifier 1403 and low noise amplifier 1404 are bidirectional, andas a result of the TDD aspect of the signal pattern employed may bereused for both transmitting and receiving signals. Local oscillator1405 coupled to mixer 1406 should be capable of switching frequencies,converting signals at either the downlink frequency F1 or the uplinkfrequency F2 to an image frequency. Optional second stage 1407 forsuperheterodyne conversion includes a filter 1408 and local oscillator1409 both tuned to the image frequency and a mixer 1410. A/D converter1208 and D/A converter 1210 are both connected to mixer 1410.

An FDD TDD strategy according to the present disclosure permitsfiltering and conversion to be performed along a single, bi-directionalsignal path which is reused for both the downlink and the uplink,eliminating the need for separate paths and reducing the system costs.The spectral performance illustrated in FIG. 13 should be implemented byfiltering system 1400, with the frequency switching time for localoscillator 1405 within the first conversion stage being critical tomeeting the timing requirements imposed by the FDD TDD system of someembodiments of the present disclosure.

It is important to note that while the present disclosure has beendescribed in the context of a fully functional data processing systemand/or network, those skilled in the art will appreciate that themechanism of the present disclosure is capable of being distributed inthe form of a computer usable medium of instructions in a variety offorms, and that the present disclosure applies equally regardless of theparticular type of signal bearing medium used to actually carry out thedistribution. Examples of computer usable mediums include: nonvolatile,hard-coded type mediums such as read only memories (ROMs) or erasable,electrically programmable read only memories (EEPROMs), recordable typemediums such as floppy disks, hard disk drives and CD-ROMs, andtransmission type mediums such as digital and analog communicationlinks.

Although the present disclosure has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the disclosure in its broadest form.

What is claimed is:
 1. A method for use in a wireless network comprising a base station and a plurality of subscriber stations in wireless, radio frequency (RF), time division duplex (TDD) communication with the base station, the method comprising: for each of the subscriber stations, establishing a plurality of associated RF connections between the base station and the subscriber station; monitoring communication traffic on the connections; setting a modulation format for each of the associated RF connections from the base station to the associated subscriber station based upon the monitored communication traffic; assigning each of the associated RF connections to at least one of a plurality of modulation groups based on a combination of at least the modulation format for the connection and a beam forming for the connection; transmitting signals on associated RF connections assigned to a first of the plurality of modulation groups in a first group of adjacent TDD slots within a TDD frame; and transmitting on associated RF connections assigned to a second of the plurality of modulation groups in a second group of adjacent TDD slots within the TDD frame.
 2. The method of claim 1, further comprising setting, by the base station, for each of the connections the beam forming for the connection.
 3. The method of claim 2, wherein the beam forming for each of the connections is set based upon the monitored communication traffic.
 4. The method of claim 3, wherein the beam forming for each of the connections is set based upon error rates detected in the monitored communication traffic.
 5. The method of claim 1, further comprising setting, by the base station, for each of the connections a forward error correction for the connection.
 6. The method of claim 5, wherein the forward error correction for each of the connections is set based upon the monitored communication traffic.
 7. The method of claim 6, wherein the forward error correction for each of the connections is set based upon error rates detected in the monitored communication traffic.
 8. A wireless network comprising a base station and a plurality of subscriber stations in wireless, radio frequency (RF), time division duplex (TDD) communication with the base station, the wireless network comprising: a set of one or more hardware processors configured to: for each of the subscriber stations, establish a plurality of associated connections on a RF communication link between the base station and the subscriber station; monitor communication traffic on the connections; set for each of the connections a modulation format of the RF communication link from the base station to the associated subscriber station based upon the monitored communication traffic; assign each of the connections to at least one of a plurality of modulation groups based on a combination of at least the modulation format for the connection and a beam forming for the connection; cause transmission of signals on connections assigned to a first of the plurality of modulation groups in a first group of adjacent TDD slots within a TDD frame; and cause transmission of signals on connections assigned to a second of the plurality of modulation groups in a second group of adjacent TDD slots within the TDD frame.
 9. The wireless network of claim 8, wherein the set of one or more hardware processors are further configured to set for each of the connections the beam forming for the connection.
 10. The wireless network of claim 8, wherein the beam forming for each of the connections is set based upon the monitored communication traffic.
 11. The wireless network of claim 8, wherein the beam forming for each of the connections is set based upon error rates detected in the monitored communication traffic.
 12. The wireless network of claim 8, wherein the set of one or more hardware processors are further configured to set for each of the connections a forward error correction for the connection.
 13. The wireless network of claim 8, wherein the forward error correction for each of the connections is set based upon the monitored communication traffic.
 14. The wireless network of claim 8, wherein the forward error correction for each of the connections is set based upon error rates detected in the monitored communication traffic. 